drm/i915: Large page offsets for pread/pwrite

Handle integer overflow when computing the sub-page length for shmem
backed pread/pwrite.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181012140228.29783-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -1127,11 +1127,7 @@ i915_gem_shmem_pread(struct drm_i915_gem_object *obj,
 	offset = offset_in_page(args->offset);
 	for (idx = args->offset >> PAGE_SHIFT; remain; idx++) {
 		struct page *page = i915_gem_object_get_page(obj, idx);
-		int length;
-
-		length = remain;
-		if (offset + length > PAGE_SIZE)
-			length = PAGE_SIZE - offset;
+		unsigned int length = min_t(u64, remain, PAGE_SIZE - offset);
 
 		ret = shmem_pread(page, offset, length, user_data,
 				  page_to_phys(page) & obj_do_bit17_swizzling,
@@ -1575,11 +1571,7 @@ i915_gem_shmem_pwrite(struct drm_i915_gem_object *obj,
 	offset = offset_in_page(args->offset);
 	for (idx = args->offset >> PAGE_SHIFT; remain; idx++) {
 		struct page *page = i915_gem_object_get_page(obj, idx);
-		int length;
-
-		length = remain;
-		if (offset + length > PAGE_SIZE)
-			length = PAGE_SIZE - offset;
+		unsigned int length = min_t(u64, remain, PAGE_SIZE - offset);
 
 		ret = shmem_pwrite(page, offset, length, user_data,
 				   page_to_phys(page) & obj_do_bit17_swizzling,